Given these findings, the researchers believe that sleep is important to building and maintaining memories. "Sleep is not an inactive state. That's an obsolete concept," Ellenbogen said. "The brain is doing lots of things during sleep, including consolidating memories. So you need to get sleep on a regular basis in order to maximize memory."

One expert thinks this study shows that sleep is important in learning.

"Sleep specialists still do not know the overarching purpose of sleep," said Dr. Robert D. Vorona, an associate professor in the Division of Sleep Medicine at Eastern Virginia Medical School, in Norfolk. "However, we do know that insufficient sleep is associated with negative alterations in both mood and performance."

A number of studies suggest that sleep plays an important role in effective memory acquisition, Vorona said. "This study suggests that parents of students would do well to recommend that their children both study hard and obtain sufficient sleep in order to maximize their academic performance," he added.

More information

The U.S. National Heart, Lung, and Blood Institute can tell you more about sleep.

Apathy Without Depression a Sign of Parkinson's

Parkinson's disease patients can be apathetic without being depressed, and apathy may be a core characteristic of Parkinson's, U.S. research shows.

"This study shows that it's important to screen for both apathy and depression so patients can be treated appropriately," study author Lindsey Kirsch-Darrow of the University of Florida, Gainesville, said in a prepared statement.

Reporting in the July 11 issue of Neurology, the Florida researchers compared 80 people with Parkinson's to 20 people with another movement disorder called dystonia.

They found that 51 percent of the Parkinson's disease patients exhibited strong signs of apathy, compared to 20 percent of the dystonia patients. Apathy without depression was noted in 29 percent of the Parkinson's patients and none of the dystonia patients. Both groups had similar rates of depression.

Apathy and depression share many of the same symptoms, which means they can be misdiagnosed, the researchers noted. Loss of motivation, loss of interest, loss of effortful behavior, neutral mood, and sense of indifference are among the characteristics of apathy.

Kirsch-Darrow believes it's important to educate family members and caregivers about apathy to help them understand that it is a characteristic of Parkinson disease. "Apathetic behavior is not something the patient can voluntarily control, and it is not laziness or the patient trying to be difficult -- it is a symptom of Parkinson disease," she said.

In an accompanying editorial, neurologist Dr. Irene Richard, of the University of Rochester in New York, noted that the current criteria for diagnosing depression may not be appropriate for Parkinson's patients.

"A person with Parkinson disease might be diagnosed with minor depression based solely on the presence of apathy. The recognition that apathy can present without depression is important so that we do not inappropriately diagnose and treat a depressive disorder that is not present," Richard wrote.

More information

The U.S. National Institute of Neurological Disorders and Stroke has more about Parkinson's disease.